Most of the overflow structures in Flanders are 'high side weir overflows'. For the design of these structures British guidelines are used. However, the design conditions for sewer systems in the U.K. differ from the Flemish design conditions. In Flanders, pressurised flow is in common use in the pipes of the sewer system. That is why tests have been carried out on a scale model of a high side weir overflow. The optimal dimensions are determined, using different model sediments. Also efficiency relationships have been determined. For a particular chamber geometry and particular flow conditions, the efficiency of a certain sediment fraction is only determined by the settling velocity of this fraction. (C) 1999 IAWQ Published by Elsevier Science Ltd. All rights reserved